The present invention pertains to the manufacture of roller-cone-type rock bits generally in the manner described in U.S. Pat. No. 4,711,143. To facilitate understanding of the present disclosure, U.S. Pat. No. 4,711,143 is expressly incorporated herein by reference.
Briefly, the type of bit in question typically includes a body having a tool joint at one end for connecting the bit to the drill string. The other end is typically trifurcated, i.e. including three legs extending longitudinally generally in the opposite direction of the tool joint, radially displaced from the centerline of the bit, and circumferentially spaced from one another. At its outermost (lowermost in use) end, each leg has a trunnion extending angularly therefrom generally in a radially inward direction, and a respective roller cone is rotatably mounted on each of these trunnions. The number of legs, and thus the number of cones, can, of course, vary, but the vast majority of such rock bits include three legs and three cones.
Traditionally, probably the most common way of forming the main body of such a bit was to form three segments each extending the full length of the bit body, and each including a respective one of the legs as well as a one-third arcuate segment of the tool joint, and then to weld these segments together along joining lines extending generally longitudinally along the bit body. See U.S. Pat. No. 4,276,946 and U.S. Pat. No. 4,187,743 for examples of the forms of such segments.
There had been, however, other techniques in which a portion of the bit body including the tool joint and three stub-like portions of the legs would be formed, the remainders of the legs separately formed, and then joined to the stubs along generally horizontal or transverse surfaces. See U.S. Pat. No. 4,158,973.
Improvements over both of these prior methods were achieved by the technology described in the aforementioned U.S. Pat. No. 4,711,143. A main body member was provided, having a tool joint at one end and a plurality of leg portions or stubs at the other end. Such a body could be of new manufacture, or could be salvaged from a used bit. However, the outer surfaces of the leg stubs to which leg extensions would be welded were not straight horizontal, but were inclined longitudinally inwardly from their radially outer extremities to their inner extremities, preferably lying on a common conical locus. The end surfaces of the leg extensions which were welded to these outer surfaces of the leg stubs were correspondingly shaped.
A number of disadvantages have been encountered in forming the legs of such bits, both in the context of the traditional three segment method, and in the context of the last-mentioned technique wherein leg extensions are welded transversely onto leg stubs. Many of these disadvantages revolved around the machining of the trunnion, which typically includes at least one annular bearing surface and an annular, concave ball race. It is, of course, desirable that the bearing surface, and to a certain extent even the ball race, be nicely finished and otherwise adapted to reduce friction in use.
In the traditional tri-segment method of rock bit manufacture, the workpiece on which the trunnion must be formed typically has a length equivalent to the length of a complete bit body, including the tool joint. With such a large workpiece, it is, as a practical matter, impossible to form the trunnion, by turning, i.e. a machining process in which the workpiece is rotated relatively quickly while a non-rotating tool is applied to it. Because the trunnion extends angularly from the main length of the workpiece, turning would require rotation of that entire workpiece in an orientation in which the bulk of it would cause inordinate centrifugal forces, and this would exacerbate the problems already inherent in the overall bulk and weight of the workpiece.
Accordingly, in such bits, the trunnions are traditionally formed by a grinding process, in which the tool which removes metal from the work is rotated quickly, while the work is rotated slowly. As is well known in the machining arts, grinding has a number of disadvantages over turning, not the least of which are that it is time consuming and typically results in less perfect surface finish.
On the other hand, in those bit bodies in which the junctures were to run transversely, rather than longitudinally, it seemed, at least in theory, that turning might be feasible because of the ability to use a shorter workpiece. However, unexpected problems were encountered in practice. Although not nearly as long or bulky as the forgings used in traditional longitudinally joined bit bodies, the work on which the trunnion was machined had to have at least enough excess length at the end of the leg to permit it to be clamped into a traditional chuck on a lathe. The trunnion had to be rough turned, then heat treated, then returned to the lathe for fine turning or finishing. Due to the distortion experienced during the heat treating process, again related to the length of the leg extension, if the work were returned to the lathe after heat treating, and the leg extension replaced in the chuck as accurately as possible, the trunnion was no longer properly aligned with the axis of the lathe.
So difficult was this problem that it was actually found easier to remove the excess material from the leg extension, temporarily weld a tang to the leg extension, and "true" the tang to the trunnion, rather than attempt to work with the original leg extension workpiece. Later the tang would have to be removed. This procedure was, needless to say, time consuming, tedious, and difficult.
Furthermore, an inordinate number of mishaps occurred while fine turning the trunnions on those workpieces to which temporary tangs were attached.